When drilling through subterranean formations in the exploration for oil and gas, it is a common practice to drill larger diameter holes at the surface, and successively smaller diameter holes as the well is drilled deeper, cementing in tubular casings at various depths along the well bore. However, it is often desirable to drill a hole larger than the inside diameter of the last casing that was set, at some known depth below the surface. Since conventional drill bits large enough to generate the desired well bore diameter will not fit inside the casing that has already been set, special tools are used to drill a well bore diameter larger than the inside diameter of the casing. One such tool used for this purpose is an underreamer.
Underreamers typically include extendable arms that are pivotally mounted in a housing using hinge pins. The hinge pins allow for movement of the extendable arms between a retracted position and an extended position. While the underreamer is being lowered into the hole, these arms are retracted to allow the tool to pass through the inside diameter of the casing. Once at the desired depth, the arms of the underreamer are then hydraulically or mechanically actuated into the extended position, where they are used to drill a larger well bore.
Many traditional underreamer designs suffer from one or more limitations. One such limitation of previous underreamer designs has been the necessity to first drill a pilot hole with a conventional drill bit before beginning the underreaming operation. This is due to the fact that most underreamer designs cannot tolerate the shock and vibration associated with simultaneous drilling and instead disengage, returning to the retracted position.
Other underreamer designs incorporate long tubular sections that are internally tapered. Many of these designs suffer from operational limitations, as well as manufacturing difficulties, due to their quality tolerances.